1. The Field of the Invention
This invention relates to matrix resins formulations and pre-impregnated fibers and composite articles formed from said matrix resins formulations wherein the matrix resin formulations are chemorheologically tailored. The invention is also related to methods for producing and using said chemorheologically tailored resin and pre-impregnated fiber formulations to form composite articles.
2. Technical Background
Solid propellant rocket motor cases for missile systems, spacecraft boosters and other types of large and small high performance, lightweight pressure vessels are commonly made from fiber reinforcement and various formulations of poly-epoxide resins (epoxy resins) by a filament winding process. Similarly, filament winding with both polyesters and epoxy resins has made possible production of lightweight tanks, poles, piping and the like. Historically, fiberglass has been the most common reinforcement fiber. Recently other fibers such as carbon filaments, boron filaments, and high modulus organic polymer filaments, most significantly aramid filaments, have become increasingly useful in these composite structures to take advantage of their differing and sometimes unique physical properties.
The resins utilized are typically epoxy formulations based on diglycidyl ether-bisphenol A (DGEBA), reactive low molecular weight epoxy diluents, and curing agents such as aliphatic and aromatic amines and carboxylic acid anhydrides. Both flexibilized and rigid epoxy resins have been used as matrix resins for filament wound composite structures.
In providing composite articles, such as pressure vessels, either wet winding or prepreg processes have been employed. In wet winding process, the fiber is run through a resin bath containing the resin composition whereby the fiber is coated with the composition. The resulting resin-fiber combination is then wound directly into the desired structure. The structures are then cured by polymerization initiated by heat or radiation. On the other hand, if a prepreg is to be used, the fiber or "tape" is impregnated with a curable resin composition and then wound on a spool. This prepreg is stored for winding at a future time. When the prepreg is converted into a composite article, the prepreg is typically cured by polymerization initiated by heat or radiation.
The present invention provides matrix resin formulations which are especially suitable as prepreg compositions. A prepreg is composed of a reinforcing fiber and a curable resin matrix. The prepreg is generally in one of the forms referred to as tow, roving, tape, mats, fabric, broadgoods, and the like. In the past, the preparation of prepreg materials has been time consuming and expensive, especially for long-working-life prepreg. By long-working-life prepreg is meant a prepreg whose handling properties do not change significantly over thirty days in normal room handling conditions.
In order to obtain and use such long-working-life prepreg, constraints at four stages in the processing sequence must be taken into consideration. These stages include: impregnation and spooling; filament winding or lay-up; cure minimum; and ultimate or post-cure of the composite article.
During impregnation, the resin formulation must have a viscosity low enough so that it will thoroughly and evenly penetrate fiber bundles containing many thousands of filaments. For these purposes viscosities are typically under 5,000 centipoise (cp). Spooling requires high enough viscosity so that the resin does not squeeze out as the fiber is spooled. A nominal spooling viscosity for graphite fibers is generally about 1,000 cp.
Two constraints operate on resin viscosity during filament winding or lay-up. The resin must have low enough viscosity so that the prepreg conforms to, or wets the surface, minimizing interlaminar voids. Resin viscosity must be high enough that minimum viscosity during cure does not go below about 500 cp. While these constraints leave a broad range for acceptable viscosities, the cure minimum of 500 cp usually precludes the use of heat-cured resins whose room temperature viscosity is 5,000 cp or less. A resin with a room temperature viscosity of 5,000 cp would fall far below 500 cp during heated cure. Therefore, a desirable resin property is to have the viscosity rise between spooling and use.
Techniques used to cure a matrix material often temporarily reduce its viscosity. Heated cure of typical epoxy prepreg resins can reduce their viscosity by several orders of magnitude for periods of minutes to hours. If viscosity falls too low, matrix material bleeds from a curing part, compromising its reproducibility and quality. While it is important that the matrix viscosity not fall too low during cure, it is also typically important that it becomes liquid. Failure of the matrix material to melt and/or flow can be the source of void and delamination defects in composite parts.
After the cure minimum or gelatin, the chemical reactions involved in curing a matrix resin progress, raising the crosslink density and mechanical properties to the level required for use of the composite part.
Solution dilution impregnation and hot-melt impregnation techniques have conventionally been employed to prepare long-working-life prepregs. In solution impregnation, a matrix resin with a viscosity of greater than 5,000 cp is diluted with a solvent to a viscosity of less than 5,000 cp. The fiber is impregnated with this diluted resin. Solvent is then removed by heating and evaporation before the prepreg is spooled. Problems with this approach include the environmental regulation requirement that the solvent be recovered, the associated expense, and the inevitable residual solvent in the matrix resin.
In hot-melt impregnation a matrix resin with a room temperature viscosity greater than 5,000 cp is heated to a temperature where its viscosity is less than 5,000 cp. Fiber is impregnated with the matrix resin at that temperature, the prepreg is cooled, and then spooled. Problems with this approach include the need for matrix heating equipment raising viscosity of the matrix resin due to heat-induced polymerization during impregnation.
Moreover, after the resin-fiber prepreg has been spooled, in each of these processes, the prepreg must generally be stored under refrigerated conditions to prevent resin advancement and loss of tack and drape needed for processing to ensure adequate flow during the heated cure of the articles made therefrom. Such curing would prevent its use in winding or forming composite articles.
It would therefore be desirable if a matrix material or resin formulation could be provided that would go through the desired viscosity profile at room temperature and do so without requiring solvent dilution or hot-melt impregnation of fibers. It would also be desirable to spool the prepreg at room temperature immediately after impregnation of the fiber, without requiring either solvent removal or cooling of the prepreg.
It is also desirable that, after impregnation, the viscosity of the matrix rises and then levels off at a viscosity level planned for room temperature storage and later use of the prepreg. This would allow for a long term room temperature storage of the prepreg and also for a long-working-life. It would also be desirable that when the prepreg is used to form a composite article, it goes to a viscosity minimum and then gels, cures or hardens like a typical prepreg. Also, it is desirable that the matrix processing viscosity be controlled by chemical formulation rather than by solvents or heated impregnation equipment.
Such methods are disclosed and claimed herein.